1. Field of the Invention
The present invention relates to novel oriented polyester films comprising, on at least one of the face surfaces thereof, a coating for improving the bonding between films employed as base substrates for a reactive outer layer, and the composite films thus produced.
2. Description of the Prior Art
The outstanding mechanical and chemical properties of linear polyester films derived from aromatic dicarboxylic acids and glycols have resulted in their great demand for various film-forming coatings and composite materials thereof useful in a wide variety of applications: wrapping, photographic films, films or sheets for the graphic arts (printing or drafting), metallized films, and tapes for recording sounds or, in particular, images.
Despite their advantages, polyester films have always presented a problem of adhesion between the base substrate and the final coating for a given application. This weak adhesion is generally attributed to the film surface structure and to the hydrophobic nature of the polyesters in particular. Various methods have been proposed to the art in attempts to solve this problem; thus, physical (flame treatment, corona discharges, abrasive treatment) or chemical (acid treatment) treatments of the film surface, in order to improve the bonding between the base and its coating, have been widely described. Since these various treatments are not without their disadvantages, the deposition, on at least one of the face surfaces of the film, of an intermediate coating exhibiting both good adhesion to the base film and good adhesion to the specific final coating for each application has been attempted.
Many polymers or copolymers have been proposed for producing these intermediate coatings which, for reasons of convenience, will be referred to hereinafter as an adhesive primer coating or anchor coating. The polymers most suitable for improving the adhesion of the final coatings to the polyester films are acrylic in nature (cf., for example, U.S. Pat. Nos. 2,794,742 and 2,627,088; French Patent No. 1,428,831; British Patent No. 1,075,533). These polymers may be thermoplastic or crosslinkable (cf. French Patent No. 1,428,831 and British Patent No. 1,075,533). The coating of the adhesive primer is done from organic solutions or, preferably, aqueous emulsions thereof. The coating operation may be carried out before the substrate film is drawn, where appropriate between two drawing operations, or even after the drawing operation(s), although application of the primer coating before drawing or between two draws produces the best results and, in particular, the best bonding between the primer and the base film (cf. aforementioned U.S. patents).
Although the use of acrylic polymers gives rise to a substantial increase in the adhesion of the final coatings to the polyester base films, the problem of this adhesion still has not been solved to a satisfactory extent. In fact, the acrylic coating must be adapted to each type of final coating in order to produce the greatest adhesion. The consequence of this, for the industrialist, is the requirement to have access to many alternative forms as a function of the nature of the final coating and of the nature of the coating medium (influence of the polarity, or of the hydrophilic or organic nature of this medium).
It has also been proposed to use, as an adhesive primer for polyester films, aqueous solutions or emulsions of water-soluble or water-dispersible copolyesters (cf. U.S. Pat. No. 4,476,189 and European Patent No. 78,559) which are produced by polycondensation of difunctional compounds containing sulfonyloxy groups. These anchor coatings are essentially suitable for hydrophilic coatings.
As a result, serious need exists in this art for primers which improve the adhesion of the greatest possible number of final coatings without the need for extensive modification of the composition of the primer as a function of the nature of the final coating. Stated differently, this art is in great need of primers which are as universal as possible in their nature.